In a fluid transfer operation, a surface loaded with fluid (a fluid transfer surface) is in constant or repeated contact against a secondary surface, for example a printing plate, blanket cylinder, rubber roller or a target substrate surface on to which the fluid is to be applied. The fluid transfer surface usually has a precisely engineered topography which must remain unaltered over its lifetime. The surface must also be capable of repeatedly receiving and transferring a consistent and uniform quantity of fluid, and hence must be hard and wear resistant. Any damage to the surface caused by gradual wear in service or careless handling is likely to translate to defects on the intended product.
Another issue to note is that fluids to be applied to a secondary surface can be highly corrosive. Flexographic inks, for example, are typically alkaline and they sometimes contain a high content of ammonia, which will attack metals such as copper and aluminium. Furthermore, modern printing inks are complex formulations loaded with particulate filler materials, such as clay and calcium carbonate and mineral pigments, and such fillers may contribute to wear of softer printing surfaces, such as printing rolls and/or doctor blade surfaces.
Chromium electroplating (or “hard chrome”) has been used to protect (roller) surfaces against wear and corrosion by deposition of a chromium layer over the surface. However, this has its drawbacks. The deposited chromium layer includes pinpoint porosity and thus does not provide an entirely effective barrier to corrosive printing fluids. This necessitates the use of a more dense barrier film, such as nickel, deposited on the relevant surface as an undercoat before deposition of the chromium layer. Furthermore, chromium plating entails environmental and health hazards. Plating baths use chromic acid which presents an acute hazard. However, of even more concern is that plating involves the use of the hexavalent form of chromium (Cr6+) which is a human carcinogen. Spent solution must also be dealt with carefully to its high acid content and loading with heavy metals.
Since the 1970s plasma spraying of a thick layer of chromium oxide has somewhat superseded chromium electroplating as a means of imparting wear and corrosion resistance to a fluid transfer surface. Chromium oxide is extremely hard (HV˜1500) and more wear resistant than chromium plating. Following plasma spraying the surface of the chromium oxide is machined and then engraved with a uniform pattern of cells or grooves by a laser.
However, this approach is itself not without problems. The deposition efficiency of conventional air plasma spray of chromium oxide powder is relatively low (less than about 45%) and plasma spraying entails large power requirements, both of which mean that the cost of running a plasma spray system is relatively high. Furthermore, structural defects are always present in plasma sprayed coatings and porosity tends to be high. As will be appreciated, defects in the coating reduce its effectiveness as a barrier to corrosive fluids. When corrosive fluids come in contact with the underlying substrate, failure at the coating-substrate interface commonly occurs. Also, high levels porosity may restrict the cell count that can be engraved, which limits the quality of printing that can be produced.
There have also been concerns about the possibility of Cr6+ formation from thermal spraying of chromium-based powders. Indeed, in 2004 the State of California Air Resources Board passed Resolution 04-44 which stated that thermal spraying operations that use materials containing chromium may result in potentially harmful airborne concentrations of hexavalent chromium and set out control measures to address this risk.
Against this background it would be desirable to provide an alternative approach for producing fluid transfer surfaces, for example printing surfaces, that are hard, that are wear resistant and that are resistant to corrosion by fluids that will come into contact with the surface during use.